The difference in how they feel is this: gas is usually localized in one spot and comes in waves, while your baby's movements are individual kicks in different parts of your belly - they're more distinct and rhythmic. When I first felt something like movements, it was like light tapping from the inside, nothing like gas at all. Plus, fetal movements become more noticeable if you lie down and relax - gas doesn't work that way. At 16 weeks you might experience both at the same time, so don't get too hung up on pinpointing exactly what it is, just pay attention to the patterns - it'll become obvious over time.